With the widespread use of copying machines, information processing devices such as word processors and computers, and communication devices, apparatuses designed to print images by using ink-jet printing heads have been quickly popularized as printing apparatuses for outputting information from these devices. In addition, with the trend toward higher quality and colorization of visual information in the above information processing devices and communication devices, there have been increasing demands for higher image quality and colorization in printing apparatuses.
In order to meet such demands, printing apparatus are generally designed to have a plurality of printing heads for the respective color inks, i.e., cyan, magenta, yellow, and black inks, so as to cope with the trend toward colorization. To cope with the tendency toward smaller printing pixels and the like, each printing head has a printing element array formed by integrating/arraying a plurality of printing elements and pluralities of orifices and channels which are integrated at high densities.
There is, however, a certain limitation in integrating orifices and channels at high densities, and hence there is a certain limitation in reducing the size of pixels to be printed. In an image printed by such a printing apparatus, since dots forming each pixel become relatively large, an image highlight portion with a low density or the like gives a sense of graininess, posing a problem in terms of image quality.
In contrast to this, a so-called multidrop scheme is known, which reduces the liquid amount of ink droplet discharged and forms one pixel by the number of ink droplets corresponding to a printing density instead of increasing the integration densities of the ink orifices and channels, i.e., reducing the size of one pixel. According to the multidrop scheme, since the diameter of an ink dot printed on a printing sheet can be made relatively small, the sense of graininess at a low-density portion such as a highlight portion can be improved.
However, as the liquid amount of ink droplet decreases, discharging operation becomes unstable. For this reason, there is a certain limitation in reducing the ink dot size, and hence a certain limitation is imposed on the improvement of image quality. In addition, according to this scheme, as the printing gray level increases, the number of ink droplets to be discharged per pixel increases, resulting in a decrease in printing speed. This produces a contradictory relationship that as the image quality improves, the printing speed decreases.
As another method of improving image quality without increasing the orifice integration density, a halftone printing scheme using two types of inks, i.e., dense and light inks having different ink densities, is known. According to this scheme, a highlight portion is printed with light ink having a low density to make the sense of graininess due to ink dots less conspicuous, and a high-density portion is printed with dense ink. For this reason, a high-density portion can be formed without increasing the number of ink droplets unlike in the multidrop scheme, and an increase in ink droplet amount used for printing and a decrease in printing speed can be suppressed.
As pseudo-halftone processing methods using binarization processing or multilevel conversion processing, a dither method, error diffusion method, average density reserve method, and the like are known.
In the dither method, each pixel data is binarized with a threshold for each pixel determined by a dither matrix.
In the error diffusion method, for example, as described in R. Floyd & L. Steinberg, “An Adaptive Algorithm for Spatial Gray Scale”, SID 75 DIGEST, pp. 36-37, the multilevel image data of a target pixel is binarized (converted into densest-level or lightest-level data), and the difference (error) between the binarized data and the data before binarization is distributed and added to neighboring pixels.
In the average density reserve method, as disclosed in, for example, Japanese Patent Laid-Open No. 2-210962, a threshold is obtained on the basis of binary data obtained by binarizing a pixel near a target pixel or data containing data obtained by binarizing the target pixel into black or white data, and the image data of the target pixel is binarized with this threshold.
In addition, multilevel conversion processing can be done by slightly changing or correcting these binarization methods.
In these methods, however, problems arise depending on the type of image to be printed.
When, for example, a transmission image or the like which requires high grayscale quality even though it is monochrome, e.g., a chest X-ray image, in particular, the human vision resolution with respect to density increases because of the transmission image. As a consequence, even when dense and light inks are used, the density differences among the respective pixels are recognized, giving an impression of a coarse image. That is, it is required to increase the number of printing gray levels.
In the above method, in order to increase the number of gray levels for each pixel, the number of dense and light inks must be increased, and hence many multiheads are required. That is, a great increase in cost is inevitable.
As disclosed in Japanese Patent Laid-Open No. 10-324002, the present inventors therefore have proposed a grayscale printing method in which a plurality of types of inks with different densities are designed to be discharged for one color, and an ink droplet is selectively discharged a plural number of times (superimposition) for one pixel within a predetermined limit, thereby increasing the number of gray levels expressed by this pixel. According to this method, many gray levels can be expressed without greatly increasing the number of dense and light inks and the number of multiheads.
In the above grayscale printing method, however, the following problems are posed.
For example, ink stored in an apparatus for a long period of time undergoes a change in density in the path of an ink supply system owing to the influences of evaporation of water. In order to prevent this, the ink supply system may be entirely made of a material that shuts off vapor. This measure, however, demands a high cost, and hence the apparatus increases in cost.
Alternatively, a change in ink density in the ink supply system may be measured to manage the ink. According to this method, however, an ink density needs to be measured with a certain or higher precision, and hence a device for measurement is required, resulting in too much cost.
In addition, even if inks having the same density are used, a slight printing density difference is produced or printing cannot be done with an intended density (gray level) owing to a difference in characteristic (discharge amount) between printing heads. Consider a case wherein a printing head B is larger in discharge amount than a printing head A by about 5%. When ink with a dye concentration of 0.2% is discharged from the printing head A, and ink with a dye concentration of 4% is discharged from the printing head B, the density ratio of the inks themselves is 1:20, but the dye ratio on a printing medium, i.e., the density ratio, becomes about 1:21. In the above grayscale printing method, therefore, a portion with a low gray level may become higher in printing density than a portion with a high gray level (gray-level reversal).
In order to prevent such gray-level reversal, it is preferable that the actual printing densities obtained by combinations of dense and light inks used be measured, and combinations of inks to be used for the respective gray levels be determined in accordance with the measurement result. This method, however, takes much time and labor, and is not practicable.
In addition, in a printer in a ready state, ink increases in density over time as water evaporates. For this reason, even when identical images are printed, some of them may become higher in density than the remaining ones. It is very difficult to measure ink densities as absolute densities such as optical densities and maintain or manage them. Therefore, such a method has not been put into practice. Consequently, it is very difficult to predict the density of a printed image in terms of an absolute value.